Nearly half of all antibiotics given to Medicare patients appear to be prescribed for reasons other than true infections, according to the findings of a new study, which further increases concerns about the widespread use of the drugs and emergence of antibiotic-resistant “superbug” infections.

In findings published this month in the medical journal Health Affairs, researchers from the Northwestern University Feinberg School of Medicine found that many antibiotic prescriptions are inappropriate and unnecessary. However, many of the prescriptions were also prescribed outside of a doctor’s visit, suggesting that they are often given to patience with little oversight or confirmation that the drugs are medically necessary.

Researchers used Medicaid data for more than 298 million antibiotic prescriptions filled for 53 million patients from 2004 to 2013. The data indicated 45% of antibiotic prescriptions for Medicaid patients were inappropriate, meaning they were given for reasons that did not include legitimate, diagnosed bacterial infections that can be treated with antibiotics.

Nearly 20% of those prescriptions were made during doctor’s visits that did not include an infection-related diagnosis. Additionally, 28% of patients who received inappropriate prescriptions were given the prescription outside of a doctor’s visit.

Insurance claims were analyzed, not medical records, so it is unclear what the circumstances were. However, many were most likely done via a phone call, email or other communication with the doctor’s office without an actual visit into the office.

This is not the first study to warn that inappropriate and unnecessary antibiotic prescribing is occurring in nearly half of doctor’s visits, often without documented reasons, including confirmed bacterial infections.

Only 55% of Medicaid prescriptions analyzed were for doctor’s visits with an infection-related diagnosis, indicated the findings of the new study.

Non-doctor’s visit antibiotic prescriptions were less common for children than adults and were more common in the West than in other regions of the country.

Medical experts warn that inappropriate antibiotic prescribing contributes to the proliferation of antibiotic resistant bacteria or superbugs, making it harder to treat patients in the future. Some research also links unnecessary prescribing to hard to treat infections, like C. diff.

Researchers warn that since many of the prescriptions are occurring outside of doctor visits, current antibiotic stewardship policies miss at least half of antibiotic prescribing.

The healthcare system does not have practices in place to ensure high quality prescribing in the U.S. under these circumstances, researchers warn. If nearly half the prescribing is happening outside of a doctor’s office visits, those prescriptions are being missed completely. The researchers called for other steps to be taken to keep track of that prescribing.